1. Field of the Invention
This invention relates to the field of electrostatography, and more particularly, to improvements in a method and apparatus for controlling toner replenishment.
2. Description of the Prior Art
Toning or development stations for electrophotographic (EP)copiers and printers typically have two-component developer mixtures (carrier and toner). Toner depleted by toning latent images on the photoconductor must be replaced by replenishing with new toner, so that the toner concentration (TC) remains within a usable range in the toning station developer mix.
Closed-loop toner concentration control, for example see U.S. Pat. No. 4,875,078, is typically achieved by means of a TC monitor and control logic to drive a toner replenishment mechanism. TC monitors are of several types, including optical and magnetic. As noted in U.S. Pat. No. 5,649,266, one common practice is to adjust the monitor output, V.sub.MON, to 2.500V when a new load of developer at nominal 10% concentration is installed in the development station. The replenishment algorithm then acts to regulate V.sub.MON to this initial 2.500V value. Maintaining V.sub.MON =2.500V assures that TC=10% (barring monitor drift) regardless of TC monitor sensitivity.
The regulation of a replenishment control signal is made by comparing the toner monitor output V.sub.MON with a set voltage reference T.sub.ref and control toner replenishment based on the difference between T.sub.ref and V.sub.MON. In order to regulate TC to 10%, T.sub.ref has been fixed at 2.500 volts. As may be seen with reference to FIGS. 7 and 8, a toner concentration monitor voltage V.sub.MON greater than T.sub.ref indicates a toner concentration lower than aim, typically 10% as noted above, and results in the toner replenishment adding toner to the toning station.
For a given developer as toner concentration increases the toner charge to mass ratio (Q/m) decreases, and vice versa. Best system performance is observed when the toner Q/m is within a particular range. An example of a range for one application may be, for example, in the range of 17-23 .mu.C/g. Problems arise when the ratio of Q/m migrates above or below the particular range suited for the particular application. Examples of problems when the Q/m ratio is high are the tendency for transfer mottle or breakdown. Examples of problems when the Q/m ratio is low is the tendency towards excessive dusting and hollow character formation. Low values of Q/m can occur even though the developer is not considered old and need not be replaced. An additional problem faced with developers that have high variability in the Q/m ratio is that compensation needs to be made in other process parameters to obtain desirable image density. These compensations such as in primary charge level V.sub.o require commensurate broadening of the operating parameters of the system requiring more expensive components and greater difficulties in maintaining control. An additional problem is created in that measuring directly the ratio of Q/m requires additional controls and/or sensors.